1. Field of the Invention
The invention relates in general to a type of antenna, and more particularly to a type antenna that has multiple operational frequencies.
2. Description of the Related Art
The electronic industry is having its prosperity nowadays; different types of portable electronic devices are also very popular. Taking the personal digital assistant (PDA) as an example, in addition to the decreasing size of the products, the ability to do wireless transmission is also a research focus that engineers try their very best in order to obtain an competitive edge over their competitors.
In a wireless system, the antenna is the window for signal transmission and it directly influences the transmission quality of the wireless signals. Its significance is self-evident. Among the different structures of antenna, the microstrip antenna is a mature technology that (1) has simple structure, (2) has small size, and (3) can easily be integrated into circuit boards. Those properties allow microstrip antennas to play an important role in personal communicational systems. However, despite of its advantageous features, in order to realize its full potential, other objective conditions such as low dielectric constant, large current distribution, and low loss in the antenna's material need to be met. The overall quality of the antenna is closely related to these conditions.
In addition to low return loss, consideration for bandwidth is also an important factor for a good design of an antenna. In the past, designers usually increased the size of the antenna or decreased the dielectric constant of the substrate in order to achieve greater bandwidth. These old methods resulted in waste of available room in circuit boards and they are no longer viable choices due to the requirement for increasing components density in portable devices nowadays.